Problem description(s)

For some of its cheaper dedicated servers, OVH does not provide a KVM (in the virtual console sense) interface. Sometimes when a virtual console is provided, it requires a horrible java applet that won't run on modern systems without a lot of song and dance. Although OVH provides a few web based ways of installing,

I prefer to use the debian installer image I'm used to and trust, and
I needed some way to debug a broken install.

I have only tested this in the OVH rescue environment, but the general approach should work anywhere the rescue environment can install and run QEMU.

QEMU to the rescue

Initially I was horrified by the ovh forums post but eventually I realized it not only gave a way to install from a custom ISO, but provided a way to debug quite a few (but not all, as I discovered) boot problems by using the rescue env (which is an in-memory Debian Buster, with an updated kernel). The original solution uses VNC but that seemed superfluous to me, so I modified the procedure to use a "serial" console.

Preliminaries

Set up a default ssh key in the OVH web console
(re)boot into rescue mode
ssh into root@yourhost (you might need to ignore changing host keys)
cd /tmp
You will need qemu (and may as well use kvm). ovmf is needed for a UEFI bios.

    apt install qemu-kvm ovmf

Download the netinstaller iso
Download vmlinuz and initrd.gz that match your iso. In my case:

https://deb.debian.org/debian/dists/testing/main/installer-amd64/current/images/cdrom/

Doing the install

Boot the installer in qemu. Here the system has two hard drives visible as /dev/sda and /dev/sdb.

qemu-system-x86_64               \
   -enable-kvm                    \
   -nographic \
   -m 2048                         \
   -bios /usr/share/ovmf/OVMF.fd  \
   -drive index=0,media=disk,if=virtio,file=/dev/sda,format=raw  \
   -drive index=1,media=disk,if=virtio,file=/dev/sdb,format=raw  \
   -cdrom debian-bookworm-DI-alpha2-amd64-netinst.iso \
   -kernel ./vmlinuz \
   -initrd ./initrd.gz \
   -append console=ttyS0,9600,n8

Optionally follow Debian wiki to configure root on software raid.
Make sure your disk(s) have an ESP partition.
qemu and d-i are both using Ctrl-a as a prefix, so you need to C-a C-a 1 (e.g.) to switch terminals
make sure you install ssh server, and a user account

Before leaving the rescue environment

You may have forgotten something important, no problem you can boot the disks you just installed in qemu (I leave the apt here for convenient copy pasta in future rescue environments).

apt install qemu-kvm ovmf && \
qemu-system-x86_64               \
   -enable-kvm                    \
   -nographic \
   -m 2048                         \
   -bios /usr/share/ovmf/OVMF.fd  \
   -drive index=0,media=disk,if=virtio,file=/dev/sda,format=raw  \
   -drive index=1,media=disk,if=virtio,file=/dev/sdb,format=raw  \
   -nic user,hostfwd=tcp:127.0.0.1:2222-:22 \
   -boot c

One important gotcha is that the installer guess interface names based on the "hardware" it sees during the install. I wanted the network to work both in QEMU and in bare hardware boot, so I added a couple of link files. If you copy this, you most likely need to double check the PCI paths. You can get this information, e.g. from udevadm, but note you want to query in rescue env, not in QEMU, for the second case.
/etc/systemd/network/50-qemu-default.link

[Match]
Path=pci-0000:00:03.0
Virtualization=kvm

[Link]
Name=lan0

/etc/systemd/network/50-hardware-default.link

[Match]
Path=pci-0000:03:00.0
Virtualization=no

[Link]
Name=lan0

Then edit /etc/network/interfaces to refer to lan0